With the seemingly unlimited electronic capabilities afforded by the discovery of solid state electronics, the microminiturization has been extended. This has resulted in the need for various improved techniques for dissipating the heat generated by the solid state components. In modern high density semiconductor packaging technology, it is necessary to transfer heat from the surface of the heat semiconductor element to a heat sink at a rate sufficiently high to limit the increase in the temperature of the semiconductor material. The standard forced air cooling appears to have reached its limit of practicality in that the amount of air that is required to provide sufficient cooling for the limited heat dissipating surfaces of the devices introduces a noise problem, and without some auxiliary techniques cannot maintain each of a large number of components within its critical, narrow operating temperature range. In general, it is desirable to maintain the semiconductor devices at or above 20.degree. C. but at or below 85.degree. C. This temperature range or, on occasions, even a narrower temperature range is required by circuit designers to keep the operating parameters of the devices in designated operating ranges, and also to minimize the noise generated in a circuit by relatively cold devices.
Another and more recent technique has been the immersion cooling system wherein an array of components to be cooled are immersed in a tank of cooling liquid. The liquids used are the fluorocarbon liquids which have a low boiling point. These liquids are dielectric and boil at relatively low temperatures. Cooling systems using this principle are described and claimed in U.S. Pat. No. 3,774,677, U.S. Pat. No. 3,851,221, and U.S. Pat. No. 3,741,292. However, these types of modular liquid encapsulated schemes must meet certain inflexible requirements. For instance, it requires coolants of extremely high purity free of any contaminants. The liquid is in contact with the device surfaces as well as the substrates which contain metallurgy. Any impurities in the coolant is a potential source for corrosion of the metallurgy and can, therefore, reduce the operating life of the system. Another cooling technique involves providing a thermal conduction route from the device to a suitable heat sink wherein heat is thermally conducted through a suitable heat-conducting material having a relatively low thermal resistance. The technique is more commonly known as conduction cooling. Embodiments of conduction cooling are disclosed in U.S. Pat. No. 4,034,469, 4,034,468, and 3,993,123. In general, these embodiments employ a solid material in contact with the backside of a solder bonded device, and which may contact with a suitable heat sink. The path provided for heat conduction can be rigid, as in U.S. Pat. No. 4,034,469 or U.S. Pat. No. 4,034,468, or in the form of spring-biased pistons, as in U.S. Pat. No. 3,993,123. In all such techniques it is important that an intimate, large area, firm contact be made between the heat-conducting element and the device in order to reduce the usual relatively high thermal impedance that exists at the interface. Thermal greases have been employed for this purpose. However, with high-performance devices which operate at higher temperatures, this solution has not been entirely satisfactory. The thermal grease can, under certain conditions, move from the interface thereby increasing its thermal resistance and can also accumulate at undesirable locations in the package causing a potential contamination problem.
In order to meet the future cooling requirements for semiconductor packaging with a system that is dependable, inexpensive and efficient, a technique for decreasing the thermal impedance at the interface of a device and coldplate, or extension thereof, is needed.